1. Field of the Invention
This invention relates to digital programmable timers and, more specifically, to digital programmable timers for environmental control systems.
2. Description of the Prior Art
Timing controllers are used for many purposes, especially in environmental control systems for buildings. Often one function is needed in sequence or in alternation with another function, with a need for timing both functions. It is desirable to be able to control independently the duration of each timing event or function operation. Typical dual (or multiple) sequential timing controllers use two (or more) timers connected in cascade so that one is on when another is off and vice versa, with each timer independently set for duration.
Most modern timing control systems use some type of digital electronic programmable timer circuits. Although such circuits have become more readily available and more easily adaptable to various controlling functions in recent years, their cost is still considerable, justifiably in view of their internal complexity of operation. Moreover, this internal complexity of each timing controller raises questions concerning reliability and "fail-safe" operation of the system when two (or more) digital programmable timers are needed.